The September 11 attacks in New York and Washington were planned and carried out by Al Qaeda.
Osama bin Laden, the son of a wealthy Saudi businessman, had established Al Qaeda in the summer of 1988, when defeat was in sight for the Soviet Union in Afghanistan. During the Soviet war in Afghanistan, Bin Laden and Abdullah Azzam established an organization to provide support and organize Arab mujahadeen fighters. With the war winding down, Bin Laden wanted to reorient his organization for jihad operations elsewhere, including the home countries of the Arab mujahadeen. In 1989, Bin Laden returned to Saudi Arabia, but began to clash with the Saudi government which put travel restrictions on him. Bin Laden was particularly disgruntled with the Saudi regime, when it invited U.S. troops to Saudi Arabia, to defend the Kingdom during the 1991 Persian Gulf War.
In 1991, Bin Laden left Saudi Arabia for good, and ended up in the Sudan, by invitation of its leader, Hassan al-Turabi. In 1992, Al Qaeda carried out a bombing attack on a hotel in Aden, Yemen. While in the Sudan, Bin Laden was also supportive of movements in Algeria, Somalia, Egypt, Libya, and elsewhere. In 1994, Saudi Arabia officially revoked Bin Laden's citizenship, and with international pressure mounting, Sudan asked Bin Laden to leave in spring 1996.
In early 1996, the Taliban was coming to power in Afghanistan, and they welcomed Bin Laden and his organization back to Afghanistan in May 1996. Soon after Bin Laden arrived in Tora Bora, Khalid Sheikh Mohammed came to visit and presented the "planes operation" idea to Bin Laden. The idea for the "planes operation" originated with Khalid Sheikh Mohammed and his nephew, Ramzi Yousef, who was involved in the 1993 World Trade Center bombing. At the time, Bin Laden was not ready or willing to undertake such an operation, having just returned to Afghanistan. Though, Bin Laden was supporting bombing attacks within Saudi Arabia, including the Khobar Towers bombing which were carried out on June 25, 1996 on a housing complex for U.S. military personnel in Saudi Arabia. Osama bin Laden issued his first fatwa in August 1996, officially calling for jihad against Saudi Arabia.
Ayman al-Zawahiri, leader of the radical Egyptian Islamic Jihad organization, officially joined with Al-Qaeda in spring 1997. Bin Laden issued a second fatwa in February 1998, which declared war on the United States. Months later, on August 7, 1998, al-Qaeda carried out simultaneous bombings of the U.S. embassies in Nairobi, Kenya and Dar es Salaam, Tanzania. The U.S. retaliated with a limited number of missile strikes, but nothing more serious at that time. Emboldened, Bin Laden continued to plan attacks, including the 2000 bombing of the USS Cole.